1. Field of the Invention
The present invention relates to a write once type optical disc.
2. Description of the Related Art
In general, conventional write once optical discs, such as CD-R (recordable) and DVD-R, have the following structure. A transparent, resin substrate has guide grooves on one surface of the substrate. On this surface, an organic dye is applied by spin coating. Then, a metallic reflection layer, made from for example gold, silver (or silver alloy) and aluminum (or aluminum alloy), is formed on the organic dye layer. Finally, a protection layer such as an ultraviolet ray curing resin (referred to as “cover layer”) is applied on the metallic reflection layer. In order to record information on the optical disc having such structure, a laser beam for information recording is radiated on the substrate side. The laser beam decomposes the organic dye so that a refractive index changes. Heat generated by the laser beam also deforms the guide grooves and the reflection layer. As a result, pits are formed on the optical disc as recorded information.
A next generation, recordable optical disc system (referred to as “DVR” or “DVR-blue” system) has been developed recently. For the DVR (Digital Video Recorder) system, a violet laser is utilized as a light source for information recordation and retrieval, and an object lens having a high numerical aperture such as 0.85 or more is employed. A cover layer of the DVR has a thickness of about 0.1 mm, and the laser beam is radiated on the cover layer side. This suppresses an influence of optical aberration caused when the object lens having a high numerical aperture is used. This in turn increases a recording density so that a large quantity of information can be recorded on the optical disc.
A write once optical disc utilizing the DVR system is referred to as “DVR-R disc” or “DVR-blue disc”. The direction of the laser beam radiation is different from the DVR-R disc compared to the conventional write once optical disc such as CD-R disc and DVD-R disc. Thus, the accumulating direction of the layers formed on the resin substrate of the DVR-R disc is opposite that of the CD-R and DVD-R. This is sometimes called a “reverse stack” structure. Further, the DVR-R disc needs a dielectric protection layer to protect the organic dye layer from a non-cured ultraviolet curing resin during formation of the cover layer. The ultraviolet curing resin forms the cover layer. Thus, the structure and manufacturing method of the DVR-R disc are different from the CD-R disc and DVD-R disc.
As understood from the above, the DVR-R disc should be manufactured with materials and structure particularly suited for the DVR-R disc.